Brushless DC motors typically comprise a magnetic rotor and one or more stator coils. For driving the rotor by applying a suitable driving current waveform to one or more stator coils it is important to know the rotor position in relation to the stator coils. In function of this position the driver generates a current in a specific direction through a specific coil to generate torque so as to turn the rotor in a desired direction. When the rotor has turned beyond a certain position, (the commutation point), the current direction needs to be varied (commutated) so that it is again in the appropriate direction to generate torque in the desired direction.
Methods and systems for controlling the commutation of a brushless DC motor may be based on Hall sensors. These Hall sensor(s) detect the position of the rotor in relation to the stator coils and based thereon the current through the motor coils(s) is controlled.
In order to avoid the need for a Hall sensor, sensorless commutation methods are developed. Such sensorless methods may for example monitor the BEMF voltage for estimating the position of the rotor. Sensorless methods make the motor construction less complex, because the hall sensor position is critical for the operation of Hall based commutation.
In low cost high volume fan systems such as they are used for CPU cooling, refrigerator ventilation, power converter cooling etcetera, single coil fans, based on hall sensing are applied. In case in such low cost systems the hall sensor could be avoided, it is clear that the fandriver may no longer have to be applied close to the rotor, or even not inside the fan anymore. In current low cost systems remote controllers typically use PWM input signals, and FG/RD communication interface pins to control the fandrivers which are integrated into the remote fan. In case of sensorless control, a significant system simplification can be achieved by locating the fandriver close to the controller, or even integrate into the remote controller.
Another problem is that the BEMF voltage can only be measured correctly, if there is no current flowing in the coil. For this purpose, a window with no current in the coil must be created in the driving wave form profile. This might introduce a torque ripple in the torque generated by the motor, audible or EMC noise.
For example, a single-coil brushless DC motor is described in U. S. Patent Application Publication 2006/0214611 and discloses a method and circuit for controlling a sensorless single-phase DC motor that measure the back emf of the winding. In this approach, a rotor position is determined based on the back-emf by stopping the supply current and, after the current has decayed the back emf is measured.
In an alternative design disclosed in U.S. Pat. No. 9,515,588, a method of controlling a brushless permanent-magnet motor includes generating a voltage proportional to a winding voltage and generating a second signal differentiated with the first to generate a third signal that is compared to the first to create an output signal having an edge corresponding to a coincidence between the first and third signals.
In three phase brushless DC (BLDC) motors, a well-known first commutation strategy, referred to as trapezoidal control is to monitor the BEMF voltage zero crossing in the third coil which is not driven, while driving the first and second coil.
In one example design, EP 1612925 describes a synchronous acceleration control method comprising performing a phase commutation of phase current at a phase commutation time point (t0) while the BLDC motor is synchronously accelerated, detecting a phase current applying time point (t1) when a magnitude of phase current supplied to the BLDC motor exceeds a predetermined value (TH1), and controlling a voltage applying time point when voltage is applied to stator windings with respect to a rotation position of a rotator based on a transient current detection time 6t between the phase commutation time point (t0) and the phase current applying time point (t1), so that the phase current is not excessively large.
U.S. Pat. No. 9,602,032 detects rotor position based on impedance measurements and EP 2037567B1 detects rotor position based on BEMF. In more advanced three phase BLDC control strategies, referred to as sinewave strategies, the commutation is defined while all three coils are driven. There exist methods wherein at predefined moments every 60 degree of multiple of 60 degree, the rotor position is defined. In other methods, the current is continuously monitored. These methods are referred to as Field Orient Control (FOC).
As the methods get more complex, the needed calculation increases drastically. For FOC control 8-bit, 16 bit and even 32 bit CPUs are applied. Also the performance of these methods is strongly depending on the motor magnetic design, which is preferably delivering constant torque by the application of constant current. Otherwise the drive has to compensate such motor deficiencies, leading to further drive complexity.
In all sinewave methods, the essential part is to smoothly transfer the torque vector from one coil to the next with minimum torque ripple. In single-coil BLDC control, such smooth transfer is not possible, because the torque has to go through zero at the point where rotor north pole transits to a south pole.
In a single-coil motor the trapezoidal method cannot be applied because there is no undriven coil, also FOC methods are not obvious because of the strong non-linear nature of the single-coil fan torque every 180 electrical degrees.
There is therefore room for improvement for driving single-coil brushless DC motors in a sensorless way.